(NaturalNews) A new study has provided one of the strongest links yet between pesticide use and Parkinson's disease.

A team of researchers from Duke University, Miami University and the Udall Parkinson's Disease Research Center of Excellence has found that people who were exposed to pesticides were substantially more likely to develop Parkinson's disease than closely related people who did not use so many pesticides, according to a study published in the journal BMC Neurology.

Parkinson's disease is a degenerative neurological condition resulting from the damage or death of the brain cells that regulate muscular movement. When cells become unable to produce the neurotransmitter dopamine, a wide variety of problems, such as slurred speech, stiffness, tremors, and problems with balance and movement, occur. Approximately one million people in the United States - one in 300 - suffer from the disease. In the United Kingdom, approximately one in 500 people, or 120,000 people, are affected.

The researchers surveyed 319 people with Parkinson's disease on their pesticide use, and compared that use to more than 200 healthy family members and other unaffected people. The purpose of comparing relatives was to control as much as possible for genetic and non-pesticide environmental factors by looking only at people with similar backgrounds.

"Previous studies have shown that individuals with Parkinson's disease are over twice as likely to report being exposed to pesticides as unaffected individuals," lead researcher Dana Hancock said, "but few studies have looked at this association in people from the same family or have assessed associations between specific classes of pesticides and Parkinson's disease."

People who had been exposed to pesticides had 1.6 times the Parkinson's risk of people who had not been exposed, while those exposed for more than 200 days in a year had more than two times the risk. The pesticides with the strongest connection to Parkinson's disease were insecticides and herbicides. Home and garden exposure were more strongly linked to increased risk than occupational use.

Broken down by sex, men who frequently used pesticides were 2.15 times more likely to develop the disease than men who did not use the chemicals, while women were 2.43 times more likely.

The researchers also compared rates of well-water drinking and living or working on a farm between people with and without the disease. They did not find a correlation between Parkinson's disease and any of these behaviors, which are commonly used as surrogate measures of pesticide exposure. Instead, the effect turned up only when they looked directly at chemical exposure.

Scientists have known for some time that the risk of Parkinson's disease is influenced by genetics, but the gene defects that have been linked to the disease account for only a small fraction of cases. Environmental factors have also been implicated, particularly in light of the fact that the disease was mostly unknown prior to the industrial revolution but has become a common ailment since then.

Prior studies have found links between pesticide exposure and Parkinson's risk, but they have not been thought conclusive. Kieran Breen, director of research at the Parkinson's Disease Society, cites a study of 10,000 Parkinson's patients that found only 1,000 had been exposed to pesticides over the long-term. The current study, Breen said, "strengthened the fact that pesticides play a key role" in risk of the disease.

"I think there is very strong evidence now linking [pesticide exposure and Parkinson's risk,]" Hancock said. The next step, Hancock said, is to discover the biological mechanisms by which pesticides increase the risk of the disease.

The findings of another study on the links between Parkinson's and pesticides, commissioned by the British Department of Environment, Food and Rural Affairs, are expected this summer.

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